Due to the widespread use of computer, software, and network, it has become possible to accelerate and share information processing. Fundamentally, digitization of information is suited for storage, accumulation, retrieval, etc., of information, whereas a paper medium is suited for display (especially, browsability) and transfer of information. It is therefore a present state that as digitization of information is developed, the consumption of paper is increasing. On the other hand, reduction of energy consumption represented by CO2 emission is an urgent need in various fields. If a paper medium which is used for temporary display or transfer of information can be recycled, a great contribution can be made to the reduction of energy consumption. There is known a method in which a color is developed and erased by heating using a reversible heat-sensitive recording medium. However, in this method, a color-forming composition is present on a recording medium, and therefore, the method has a disadvantage that a common or plain paper medium cannot be used. It is also known that an erasable toner is produced by a pulverization method. However, the erasable toner has a disadvantage that in a process of melt-kneading components, such as a color-forming agent, a color-developing agent and a decoloring agent, the components are reacted with each other, and therefore, the density of a developed color is decreased and also a decoloring reaction rate is decreased. As a production method other than a kneading pulverization method, a production method employing a wet process in which a toner is obtained by aggregating and fusing fine particles of an erasable color material and fine particles of a binder resin, etc., in an aqueous medium is also known. According to this method, it is possible to mix the fine particles of an erasable color material with the binder resin, etc., to effect coalescence without being subjected to mechanical shearing or high thermal history by melt-kneading. However, in this method, it is not easy to completely incorporate color material fine particles in toner particles, and this method has been found to involve a problem that fine particles of the color material released from the toner particles remain in the toner as fine powder to cause an image defect, such as fogging. This tendency is particularly pronounced when the color material fine particles are microencapsulated.
On the other hand, it is also known to crosslink a toner resin using a reactive polymer, so as to mainly improve the fixability, thermal characteristics and mechanical characteristics of a toner.